In the past decades, accumulating knowledge in stem cell biology has exerted extensive impacts on the understanding of human ontogenesis and homeostasis, and particularly of the genesis of human malignancy and the related therapies (see e.g., Bonnet and Dick, 1997). Stem cells have been shown to be present in a variety of mammalian tissue systems, including skin, gut, central nervous system, and hematopoietic systems (see e.g., Blanpain et al., 2004). Stem cells are centrally defined by the capabilities of self-renewal, differentiation into a broad spectrum of specific lineages and long-term proliferation. In view of the analogies between normal stem cells and tumorigenic cells in terms of their physiological features, tumorigenic cells have been recognized as tumor stem cells, and there are indeed increasing documentations confirming the existence of tumor stem cells in hematologic cancers, central nervous system tumors and breast tumors (see e.g., Bonnet and Dick, 1997; Cobaleda et al., 2000; Jordan et al., 2006).
Tumor stem cells distinguish themselves from normal stem cells and classically-defined tumor cells. As compared with the normal counterparts, tumor stem cells display certain unique physiological characteristics. For example, leukemia stem cells exhibit abnormal activation of nuclear factor kappa B and elevated expression level of CD123, interferon regulatory factor 1 (IRF-1) and death-associated protein (DAP) kinase. Unlike the classically-defined tumor cell populations, tumor stem cells act as parent cells thereof and are endowed with limitless self-renewal capacities and the multiple lineages-differentiating potentials.
In the field of tumor-related study and therapy, emerging experimental and clinical data have shown that many tumors arise from a rare population of cells, namely tumor stem cells (see e.g., Jordan et al., 2006). Tumor stem cells are oncogenic, poorly differentiated and capable of leading to self-renewal of tumor cell populations (see e.g., Hope et al., 2004). First demonstrated was the existence of leukemia stem cells (LSCs) (see e.g., Lapidot et al., 1994). Analogous to normal hematopoietic stem cells (HSCs), LSCs comprise only a small fraction of leukemia cells, belong to the top of hierarchical organization with poorly differentiated status and display limitless self-renewal capacity. They account for the genesis of leukemia progenitor cells with limited proliferation capacity and downstream leukemia cells at different differentiation degrees, and consequently lead to the onset of leukemia. Therefore, in targeted therapies for tumors, tumor stem cells represent a promising target for drug design and screening.
Most conventional anti-tumor agents act on those mature differentiated tumor cells downstream the tumor stem cells, and are extremely insensitive to those poorly differentiated tumor stem cells having limitless proliferation potentials. Thus, most tumor patients, although can be clinically alleviated, frequently suffer from drug resistance and tumor relapse due to the lack of effective drugs specifically targeting tumor stem cells. For example, chronic myelogenous leukemia (CML) is a CML stem cell-originated disorder (see e.g., Huntly and Gilliland, 2004), characterized mainly by the oncogenic BCR-ABL fusion protein resulting from gene translocation, thereby exhibiting abnormal tyrosine kinase activity. A well-designed inhibitor targeted to this tyrosine kinase, imatinib (STI571, Gleevec), can effectively kill leukemia cells (see e.g., Deininger et al., 2000). However, many CML patients often suffer from drug resistance and relapse following long-term of imatinib administration. Molecular diagnosis shows that BCR-ABL positive cells remain in almost all the patients, suggesting that tyrosine kinase targeted inhibitor imatinib alone cannot eliminate all of the leukemia cells, especially leukemia stem cells (see e.g., Graham et al., 2002). Another example is acute myelogenous leukemia (AML), characterized by premature arrest of myeloid development and the subsequent accumulation of large numbers of non-functional leukemic blasts, including AML stem cell population (see e.g., Bonnet and Dick, 1997). Since the persistence of surviving AML stem cell population after the conventional chemotherapy, like cytosine arabinoside (Ara-C) that mainly kill dividing cells through interfering with DNA replication (see e.g., Guan and Hogge, 2000), it is always hard to achieve durable and complete remission (see e.g., Guzman et al., 2002; Guan et al., 2003).
Thus, there apparently is an urgent need for therapeutic strategies, specifically ablating tumor stem cells while sparing normal stem cells, which will not only overcome the drug resistance of tumor and the tumor relapse, but also bring great benefits for prevention and permanent cure of tumor disease originating from tumor stem cells.
Surprisingly, the inventors have found that, fenretinide or bioactive derivatives thereof can specifically induce the apoptosis of primitive tumor cells, particularly quiescent CD34+CD38− leukemia stem cells, and can also kill CD34+CD38+ leukemia progenitor cells having similar physiological features to leukemia stem cells, while having no significant effect on normal hematologic stem cells. The inventors have further found that combination therapy associated with fenretinide or bioactive derivatives thereof can achieve better therapeutic effect. Moreover, the inventors have established a method for effectively screening anti-tumor agents that can combine with fenretinide or bioactive derivatives thereof to induce apoptosis of tumor cell, especially hematologic tumor cells.